The human-body shadowing problem has been a significant issue for millimeter waves (mmWaves) in the fifth-generation communication system. Using a passive reflector to create alternative paths has been studied as a shadowing mitigation strategy. This paper analyzes the shadowing effect at mmWave using the full-wave analysis based on the method of moment in a two-dimensional environment. It is found that a maximum improvement in the shadowing region is more than 30 dB when a passive reflector is introduced.

This research focuses on multiple-input multiple-output signal separation in environments with changing channels, such as mobile communications. When separating signals by combining weights fixed in a packet, the orthogonality between interfering signals and combining weights is broken due to the movement of the receiver, resulting in interference leakage. To mitigate such degradation, a null-space expansion technique in the frequency-domain is employed, assuming that there are extra antennas at the receiver side. This technique improves channel time-varying immunity by using extra receiving antennas to form a null-space that suppresses interference between streams. In this paper, the performance advantage of the null-space expansion technique is revealed through in-lab and field measurement campaigns.

The terminal-collaborated multiple-input multipleoutput (MIMO) reception technology can equivalently increase the number of receiving antennas by collaborating the receiving terminals. Using this technology, the spectrum efficiency of communication between the base station and the collaborating terminals can be increased. It is desirable to use high-frequency bands with high-speed and low-latency for collaborative communication. In this paper, we report the initial implementation results of a transmission experiment system for MIMO reception with terminal collaboration that uses the 25.9 GHz band for terminal-to-terminal communication, with the goal of future outdoor experiments. As a result, we show that the error rate performance is improved as the number of collaborating terminals is increased.

This research focuses on multiple-input multiple-output signal separation in environments with changing channels, such as mobile communications. When separating linear signals by fixing combining weights to packets, the orthogonality between interfering signals and combining weights is broken due to the movement of the receiver, resulting in interference leakage. To suppress such degradation, we use the null-space expansion technique in the frequency-domain, assuming that there are extra antennas at the receiver side. This technique improves channel time-varying immunity by using extra receiving antennas to form a null space that suppresses interference between streams. Experiments evaluated the performance of the fluctuation tolerance when combining weights were applied to the entire packet.

The use of high-frequency bands is becoming increasingly popular for achieving higher speed, lower latency, and higher capacity in wireless communication systems. One way to use high-frequency bands is to improve the transmission performance by linking neighboring terminals. However, it is recognized that high-frequency band wireless communications are susceptible to obstructions. In particular, the shadowing effect caused by the human body between neighboring terminals is a major problem, especially when high-frequency bands are used for terminal collaboration. This paper reports the comparison results of the shadowing effects measured in the super high frequency and ultra high frequency bands for a reciprocating motion pedestrian case. In this research, experiments are conducted in the electromagnetic anechoic chamber room to measure the effects of human shadowing in the $25.9 \mathbf{G H z}$ and 438 MHz bands. As a result, it was confirmed that the 25.9 GHz band has a shorter shadowed time and smaller shadowing gain than the 438 MHz band.

The transmission performance deteriorates if the combining weights for interference suppression are fixed within a packet during signal separation in linear MIMO reception, since the propagation path changes with time in mobile communications. To solve this problem, we apply the null-space expansion technique, which improves transmission performance by forming additional nulls to suppress inter-stream interference using antenna degrees of freedom. In this study, we consider acquiring the past channel information for the null-space expansion technique from multiple positions in a packet. Error rate performance is evaluated using computer simulations when combining weights generated by this technique are applied to the entire packet.

This paper deals with a terminal-collaborated MIMO reception technique, in which nearby terminals collaborate to receive MIMO signals from the base station. This system can improve the transmission performance because the number of received signals used for decoding can be expanded by sharing the MIMO signals from the base station among nearby mobile terminals. In this paper, we focus on the fact that the amount of traffic for transmission of the decision result is generally smaller than that of the received signal waveform. We assume multiple helper terminals that forward waveforms within a collaboration group and multiple detection terminals that perform decoding. In this system, there is a possibility that the degradation of decoding performance caused by decreasing the number of received signal waveforms transmitted can be improved by exchanging decision results among the detection terminals. The residual interference coefficients in frequency-domain iterative equalization are employed as important information for the reliability of the decision result. The decoding performance is improved by selecting the decision result with high reliability as the final decoding results. We evaluated the effects of the number of helper terminals and the number of detection terminals on the transmission performance and the amount of collaboration traffic by computer simulations. As a result, the improved transmission performance was confirmed with a slight increase in the amount of collaboration traffic.

We are developing a collaborative multiple-input multiple-output reception system consisting of a base station (BS) with multiple antennas and surrounding multiple mobile stations (MSs) with a small number of antennas for future cellular systems. The MSs virtually increase the number of receiving antennas by forwarding the received waveform data using higher-frequency bands and decode the multiple received streams from the BS. The multiple received spatially multiplexed streams experience quantization errors, which are induced by digitized waveform forwarding. We obtain experimental results for the mutual information rate by conducting a measurement campaign around an urban area, in which channel state information is measured on a moving vehicle.

This paper considers the performance improvement of MIMO signal separation using a linear spatial filter in a time-varying channel such as in a mobile environment. The transmission performance deteriorates due to the movement of the receiving station if the combining weight that suppresses interference signals in a packet during linear signal separation is kept fixed. In this paper, the null-space expansion technique is applied to suppress such degradation. This technique improves the robustness against the time-varying channel by additionally forming nulls that suppress inter-stream interference by utilizing the antenna degrees of freedom. The robustness against intra-packet channel fluctuation is studied by computer simulations and the improvement of error ratio performance is confirmed.

Multiple-input multiple-output (MIMO) collaborative reception is an emerging technology in wireless communications. In this reception system, received signals from a base station are shared among neighboring multiple mobile stations (MSs). Multiple received signals can be used for decoding the MIMO signal, so the detection performance is enhanced. In particular, we consider collaborative detection schemes in order to improve the detection performance further while keeping the collaboration traffic as small as possible. Collaborative detection schemes utilize multiple decision results of multiple detection MSs. Through computer simulations, we show that a selection scheme based on decision reliability information outperforms soft combining schemes.

In terminal-collaborated multiple-input multiple-output (MIMO) reception, a base station transmits multiple signal streams to a virtual terminal with a large number of reception antennas. The virtual terminal consists of multiple mobile terminals sharing their signals with other terminals. As the number of terminals increases, the performance of terminal-collaborated MIMO reception improves. However, the overhead for inter-terminal communication also increases. If the optimum set of collaborated terminals is selected, it is possible to improve the performance of terminal-collaborated MIMO reception and reduce the aforementioned overhead. In this study, adaptive terminal selection schemes were examined using actual received signals previously stored during outdoor experiments.

In terminal collaborated MIMO reception, signals received from BS are shared among collaborated terminals. In this system, the performance of inter-terminal communications affects the overall MIMO reception performance. This paper investigate the performance of inter-terminal communication in vehicles. Due to increasing the distance between mobile stations or shadowing loss, signal to noise ratio (SNR) of inter-terminal communication reduces in free space model. However, SNR of inter-MS communication does not follow the free space model in vehicles, because there are multipath reflections from doors, windows, roof, and floor. This paper investigate the effect of MS arrangement, shadowing, and vehicle running in inter-terminal communication.

In this paper, the measurement results of field experiments for correlation reduction precoding with a repetition code in an overloaded MIMO systems are presented. Measurements are conducted in three propagation conditions that are line-of-sight (LOS), non-line-of- sight (NLOS), and a mobile environment. Numerical results obtained through the measurements show that amplitude shift and phase rotation precoding improves demodulation performance. It is also presented that the precoding scheme reduces bit error rate (BER) in the mobile environment.

Due to its ability to drastically improve the spectral efficiency, massive multiple-input multiple-output (MIMO) has become a strong candidate for the next wireless communication standard. Massive MIMO, however, relies its performance on the accuracy of the precoder. On the other hand, the drastic growth of traffic demand also requires the exploration of the underutilized frequency spectrum on the higher frequency bands. Unfortunately, transmission in these frequency bands will be largely affected by Doppler spread which reduces the accuracy of channel state information required by the precoder. To overcome this problem, a Doppler compensation technique for linear precoder in a massive MIMO scenario is proposed in this paper. By exploiting the channel reciprocity of time division duplexing transmission, we extract the Doppler shift information of the channel and use it to conduct the Doppler compensation by independently updating each element of the precoder matrix. Computer simulation shows that our proposed technique enables the massive MIMO transmission even when the value of maximum Doppler shift is considerably large.

Among the emerging breakthroughs for future wireless communications, massive multiple-input multiple-output (MIMO) has attracted a lot of interest. To harvest its potential, however, accurate channel state information (CSI) is required at the base station. Unfortunately, the accurate CSI is hard to be obtained when Doppler spread occurs. The mismatch between the precoder and the actual channel causes the signal to be phase-shifted, and the interuser interference arises. To overcome this problem, we propose a power allocation technique based on the predicted interference level of all users. First, we extract the Doppler shift information from the pilots sent by the mobile station (MS) to create predicted channel matrix. Based on this matrix, we conduct a recursive power allocation to minimize the interuser interference at the MSs. Computer simulation shows that our proposed technique enables the time varying massive MIMO transmission even when the precoder matrix is not frequently updated.

This paper proposes a modified bat algorithm for optimal resource allocation scheme in closed access femtocell networks. Bat algorithm is a new nature-inspired metaheuristic algorithm which is inspired by the behaviour of bats echolocation. The main objective of the proposed algorithm is to select the best combination of resource blocks for femto users so as to minimize the co- and cross-tier interference in macrocell-femtocell network. To deal with a discrete solution in resource allocation problem, the original bat algorithm updating rule is modified using nearest integer method. The simulation results show that the proposed algorithm improves the throughput of the femtocell networks compared to that of random allocation scheme.

As the demand for higher transmission rates and spectral efficiency is steadily increasing, the research and development of novel mobile communication systems has gained momentum. This paper focuses on providing a comprehensive survey of research and development activities on fifth generation mobile communication systems reported in IEICE technical committee on radio communication systems. We try to survey a vast area of wireless communication systems and the developments that led to future 5G systems.

Recently, multi-user multiple-input multiple-output (MU-MIMO) systems with spatial precoding have been widely studied. Although they offer various advantages, these systems require accurate downlink channel state information (CSI) at the transmitter and limits the mobility of user terminals. In this paper, we consider MU- MIMO systems which employ collaborative interference cancellation (CIC) between users for interference suppression. In this technique, neighboring user terminals share their received signals without decoding and equivalently increase the number of their receive antennas. The performances of the MU-MIMO systems with CIC are evaluated through field experiments for both minimum mean square error (MMSE) and maximum-likelihood detection (MLD) detection algorithms.

This paper considers the resource allocation problem for device-to-device (D2D) communication underlaying cellular networks and proposes a service-aware resource allocation (SARA) scheme for D2D communication to improve the network performance. SARA takes into account the different service requirements of D2D users. It consists of two allocation phases: on-demand resource allocation and secondary resource allocation. In the first phase, cellular user resources are allocated on demand to meet the different service requirements of D2D pairs. In the second phase, the remaining cellular user resources after the first phase are allocated to D2D users based on an objective function in order to improve the resource utilization. Simulation results show that the proposed SARA scheme can significantly improve the performance of a cellular system in terms of the satisfaction ratio of D2D pairs and the system throughput.

This paper proposes a Capacity Oriented Resource ALlocation (CORAL) algorithm for resource allocation in device-to-device (D2D) communication underlaying mobile cellular networks. To achieve high system capacity, CORAL allows a D2D pair to share the resources of more than one cellular user. Meanwhile, it introduces the concept of a Capacity-Oriented REstricted (CORE) region for a D2D pair to determine a candidate cellular user set for the D2D pair in resource allocation, which excludes those cellular users that introduce a negative system capacity gain, and thus can help increase the system capacity and reduce the computational complexity of CORAL as well. Moreover, CORAL attempts to avoid the situation where different D2D pairs simultaneously choose the same optimal cellular user, which would otherwise cause severe inteferences between the different D2D pairs and larger interference on the optimal cellular user. In addition, CORAL can ensure that each D2D pair can be allocated the resources of at least one cellular user. Simulation results show that CORAL can achieve a good performance in terms of both the system capacity and the rate loss of all cellular users.

This paper considers distributed linear beamforming in downlink multicell multiuser orthogonal frequency-division multiple access networks. A fast convergent solution maximizing the weighted sum-rate with per base station (BS) transmiting power constraint is formulated. We approximate the noncon-vex weighted sum-rate maximization (WSRM) problem with a semidefinite relaxed solvable convex form by means of a series of approximation based on interference alignment (IA) analysis. The WSRM optimization is a two-stage optimization process. In the first stage, the IA conditions are satisfied. In the second stage, the convex approximation of the non-convex WSRM is obtained based on the consequences of IA, and high signal-to-interference-plus-noise ratio assumption. Compared to the conventional iterative distributed algorithms where the BSs exchange additional information at each iteration, the BSs of our proposed solution optimize their beamformers locally without reporting additional information during the iterative procedure.

The performance of multi-user multiple-input multiple-output (MU-MIMO) systems is not only affected by multi-path fading, but also by shadowing and path-loss. In this paper, we concentrate on the sum rate distribution of MU-MIMO systems employing linear zero-forcing receivers, accounting for both multi-path fading and shadowing effects, as well as spatial correlation at the transmit and receiver sides. In particular, we consider the classical spatially correlated lognormal model, and propose closed-form bounds on the distribution of achievable sum rates in MU-MIMO systems. With the help of these bounds, we derive a relationship between the inter-user distance and channel 10% outage sum rate under different environmental features. A practical conclusion from our results is that the effect of spatially correlated shadowing can be considered as independent when the inter-user distance is approximately five times the shadowing correlation distance. A detailed analysis of the effects of composite channel attenuation consisting of multi-path fading and shadowing is also provided.

A transmission experiment of a multi-user MIMO system with user selection and linear spatial precoding is conducted and the results are reported. As a low-complexity user selection algorithm, the chordal distance-based user selection (CDUS) algorithm is employed. The testbed used in the experiment is implemented by taking full advantage of the software-defined radio. An amplify-and-forward-based simple channel estimation method is adopted. As a result of the experiment in a conference room, the CDUS algorithm improved both the power efficiency and the interference suppression accuracy compared to the round-robin user selection.

In this paper, we analyze the performance of a downlink beamforming system with partitioned vector quantization (P-VQ). A multiuser multiple-input single-output (MU-MISO) downlink beamforming system with accurate channel state information (CSI) at user terminals is considered. The optimization of feedback bits allocation to different partitions and channel direction indicator (CDI), channel quality indicator (CQI) corresponding to each partition has been performed exploiting quantization mean square error (MSE). The impacts of equal and unequal partitioning, and the number of partitions on codebook memory and overall system capacity have also been investigated and depicted through simulation results. We have found that with optimized CQI and CDI bits allocations, the amount of bits allotted to equal and unequal partitions are proportional to the size ratios of the respective partitioned sub-vectors. It has also been observed that for small-sized partitions, the optimal ratio of CDI to CQI is much smaller than that of large-sized partitions.

In this study, an adaptive and variable-rate feedback reduction technique for partitioned vector quantization (P-VQ) in MU-MISO downlink precoding is proposed. Independent feed-back reduction is performed for both channel quality indicator (CQI) and channel direction indicator (CDI) over slowly-varying Rayleigh channel. The users adaptively reduce the feedback over-head for quantized channel state information (CSI) depending on the rate and direction of change in the correlated CQIs and CDIs. Simulation results show that the proposed scheme greatly lessens the communication overhead with very small performance loss and enhances the quantization efficiency.

This paper considers coordinated linear precoding in downlink multicell multiuser orthogonal frequency-division multiple access (OFDMA) network. A less-complex, fast and provably convergent algorithm that maximizes the weighted sum-rate with per base station (BS) transmit power constraint is formulated. We approximate the nonconvex weighted sum-rate maximization (WSRM) problem with a solvable convex form by means of sequential parametric convex approximation (SPCA) approach. The second order cone program (SOCP) formulations of the objective function and constraints of the optimization problem are derived through proper change of variables, first order linear approximation and hyperbolic constraints transformation, etc. The algorithm converges to the suboptimal solution taking fewer number of iterations in comparison to other known iterative WSRM algorithms. Finally, numerical results are presented to justify the effectiveness and superiority of the proposed algorithm.

In order to demonstrate the channel capacity improvement by multiple-input multiple-output (MIMO) transmission techniques in distributed antenna systems (DAS), the channel capacities of both centralized antenna systems (CAS) and DAS are compared assuming spatially correlated shadowing. Many studies deal with these channel capacities assuming spatially independent shadowing. In this paper, we compare the characteristics of MIMO channel capacity in CAS and DAS under the effect of distance-dependent path loss, spatially correlated shadowing loss, Rayleigh fading, and inter-cell interference. Computer simulation results indicate that DAS-MIMO transmission can significantly reduce the spatially correlated shadowing loss, thus improving the outage capacity compared to CAS-MIMO transmission. Furthermore, the simulation results also show a relationship between inter-antenna distance and channel capacity.

A practical and efficient vector quantization called partitioned vector quantization (P-VQ) based non-linear precoder for MIMO downlink broadcasting channels has been analyzed in this paper. P- VQ has been found to be an efficient way of reducing the memory requirements and search complexity in conventional VQ systems, especially for large MIMO. Simulation results reveal that P-VQ technique can enhance the overall quantization performance in terms of achieved capacity, bit error rate (BER) at a cost of reasonable additional complexity under certain feedback budgets. Tomlinson-Harashima precoding (THP), a power and complexity efficient precoding technique along with data dependent vector perturbation (VP) has been employed to meet the power constraint requirements of our system. Least-square (LS) channel estimation is performed at the user terminals to acquire their channel state information (CSI).

In this paper, the end-to-end packet error rate (PER) performance of a space-time block code (STBC) based multi-hop cooperative relaying system is discussed. This system consists of a source, a destination, and two STBC based cooperative relays in each hop. These relays decode their received packets and forward the packets only when no error is found. In this system, there are conditions where the end-to-end PER performance improves with the number of hops. The theoretical end-to-end PER performance of this system have been derived analytically. However, in real-world wireless systems, various practical issues, e.g., timing synchronization and imperfect channel estimation, have a large impact on the performance. To confirm the end-to-end PER performance of this system in actual setup, an in-lab experiment using a fading emulator and four transceivers is performed under Rayleigh fading environment. The experimental results clarify the PER performance of the STBC based multi-hop cooperative relaying system. In addition, the theoretical PER performance is also verified from the experimental results.

Adaptive traffic ratio control (ATRC) with hybrid network coding and cooperative relaying is proposed to improve the overall throughput in an asymmetric data traffic network. The scheme is used in conjunction with adaptive transmission protocol and relay selection based on instantaneous channel state information. Simulation results show that (i) ATRC provides better trade-off between achieved throughput and traffic ratio and (ii) both maximum throughput and guaranteed traffic ratio can be simultaneously achieved by ATRC as offered traffic approaches symmetric.

In this paper, implementation of a multi-user MIMO experimental system is presented. Linear spatial precoding and a simple two- way channel estimation technique are adopted in the experimental system. In-lab and field transmission experiments are carried out and the performance is evaluated. The impact of channel estimation error under average channel gain discrepancy between two mobile stations is analyzed by means of computer simulation. It is shown that channel estimation error has a greater influence on the mobile station with greater average channel gain.

In this paper, we propose a receiver using the concept of a "virtual channel" and Turbo codes for multiuser multiple-input multiple-output (MU-MIMO) systems with cooperating transmitters. In order to reduce feedback information from receivers, a precoding method with reduced channel state information (CSI) is needed. Although signals interfere with each other in such a system, the proposed receiver enables high-speed transmission by detecting signals iteratively. In the proposed iterative scheme, soft information is exchanged between virtual channel detectors and Turbo decoders to improve the bit error rate (BER) performance. We evaluate the performance of the proposed receiver in systems with two transmitters, each equipped with four antennas, and two receivers, each equipped with two antennas. The results show that the proposed receiver can improve the system performance owing to the iteration scheme.

In multi-hop cooperative communications, multiple relay stations are used at each hop in order to enhance transmission performance. In previous studies, transceivers have been used to evaluate the transmission performance of multi-hop cooperative communications involving the use of a space-time block code, in real propagation channels. In the present study, an experiment involving multi-hop cooperative networks is conducted with transceivers. The transceivers are implemented on field programmable gate arrays, which perform real-time signal processing. Synchronization of the timing and carrier frequency among wireless stations is carried out on the basis of received signals. The transmission performance of cooperative communications is compared to the performance of non-cooperative multi-hop communications involving the use of only one relay station. The performances of the relay schemes are measured quasi-simultaneously. The transmission performance is evaluated in terms of the cumulative distribution function (CDF) of the bit error ratio.

This paper proposes a game-theoretic approach for self-organized resource allocation in OFDMA femtocells with closed access configuration. The objective of the proposed scheme is to overcome the interference problem caused by co-channel operation of femtocells in an existing macrocell network using a self-organization capability of femto users. Inspired by the emerging cognitive radio technology, each femto user acts as an autonomous entity and attempts to select the most appropriate subset of resource blocks in a decentralized manner in order to mitigate the cross- and co-tier interference. Such a self-organized resource allocation scheme can be modeled as a potential game, which guarantees the convergence to a Nash equilibrium as long as distributed sequential play based on the best response strategy is adopted. Simulation results show that the proposed scheme improves the capacity of the femtocell network, while minimizing the performance degradation of the macrocell network.

With the purpose of improving the performance of next generation wireless networks, cooperative relaying (CoR) and network coding (NC) are promising techniques for bi-directional or two-way transmissions. Compared to CoR, NC can reduce the number of bi-directional transmission slots, which results in higher total throughput. However, the asymmetric traffic condition has significant impact on the bi-directional throughput of NC. CoR is robust against asymmetric traffic conditions when adaptive transmission direction switching (TDS) based on channel and traffic condition is applied. In order to improve the throughput of NC even under asymmetric traffic condition, two hybrid NC and CoR schemes with opportunistic scheduling are proposed in the present paper. In the proposed schemes, adaptive resource allocation is realized for each transmission protocol, and the impact of the traffic ratio and the relay position on the throughput is investigated. Simulation results reveal that the proposed schemes not only achieve higher throughput than the conventional scheme but are also robust against asymmetric traffic conditions. Moreover, the results can contribute to the development of relay selection and routing schemes for practical networks.

In this paper, we propose a virtual layered space-frequency receiver with an ordered successive detector for multiuser multiple-input multiple-output (MU-MIMO) systems where multiple transmitters cooperate with one another. The proposed receiver enables high-speed transmission in the systems even if the transmitters know only partial channel state information (CSI). Actually, the receiver shows such superior performance when the concept of "virtual channel" is applied to cooperative MU-MIMO systems. Moreover, we propose an iterative detection scheme for the receiver; in this scheme, some signals from the Viterbi decoder are fed back to the ordered successive detector. The proposed iterative scheme helps improve the transmission speed when the number of cooperating transmitters is increased. We evaluate the performance of MIMO systems in which the transmitters have four antennas each and two users have two antennas. The receiver can double the transmission speed at the cost of 3 dB in Eb/N0at a BER of 10-4. Furthermore, the receiver can triple the transmission speed at the cost of 7 dB in Eb/N0at a BER of 10-4. When the system has three transmitters, we observe an error floor. However, the proposed iterative method can suppress the error to a value below BER = 10-7.

In cognitive radios, reliable detection of wireless systems is crucial since these radios share the frequency spectrum with other wireless systems. In a heterogeneous environment, systems with different priorities have different protection requirements and operating parameters.The cognitive radio must guarantee highly reliable communication in the primary systems, but need not necessarily guarantee reliable communication in the secondary systems. Therefore, classification of the signals in a cognitive radio is necessary to fulfill the above mentioned protection requirements. Cyclostationary detection allows for the classification of coexisting cyclostationary signals with different parameters, whereas energy detection cannot be used for distinguishing primary signals from secondary signals. This method has been theoretically investigated, but it has not been applied to an actual field. This paper presents the experimental results obtained from field trials on the use of cyclic-feature-based methods for distributed signal detection and signal classification.

Recently, the concept of self-organization has drawn considerable attention for its possible use in distributed interference management. To realize self-organized interference management, a minority game (MG) is a promising tool because it facilitates self-organized decision-making. Although the existing interference management scheme that involves the use of an MG achieves transmission control aware of the system priority, it cannot actively control the outage probability of a higher-priority system. In the present study, the existing scheme is modified to control the outage probability for a predefined target value. It is proposed that the criterion for deciding whether or not to transmit should be changed adaptively, depending on the interference power in the higher-priority system. In this way, robustness against the location of stations is also achieved. Numerical simulation confirms the above advantage of the proposed scheme.

In multi-hop cooperative communications, multiple relay stations are used at each hop in order to enhance transmission performance. Transceivers for multi-hop cooperative communications using space-time block code are developed in order to evaluate the transmission performance in real propagation channels. In the present paper, an experiment involving multi-hop cooperative networks is conducted with this equipment. The transceivers are implemented on field programmable gate arrays, which perform real-time signal processing. Synchronization of timing and carrier frequency among wireless stations is established based on the received signals. The transmission performance of cooperative communications is compared to the performance of non-cooperative multi-hop communications using one relay station. The performances of the relay schemes are measured quasi-simultaneously. The performance is evaluated in terms of the cumulative distribution function (CDF) of the bit error ratio.

In the present paper, a game theoretic framework of joint channel selection and power allocation for spectrum sharing in distributed cognitive radio networks is proposed. The objective of the proposed scheme is to investigate the performance of the lower- priority system in spectrum sharing where the lower-priority users utilize the spectrum by selecting the channel and transmit power while aware of the interference to the higher-priority users. The utility function that captures the cooperative behavior to minimize the interference and the satisfaction to improve the throughput is defined. The proposed joint channel selection and power allocation game can be formulated as a potential game and it is guaranteed to converge to a Nash equilibrium when the best response dynamic is performed. The simulation results verify the convergence of the proposed potential game and reveal that the proposed utility function with coefficient adjustment improves the throughput performance of the lower-priority system compared to that of without coefficient adjustment, while ensuring the Quality of Service (QoS) constraint at the higher-priority system.

Adaptive radio resource management policy aware of the demand for wireless services is termed dynamic spectrum access (DSA). In order to evaluate the effectiveness of DSA, dynamics of the demand should be considered. In the present paper, dynamics of the spectrum market, which is a way to realize DSA, are evaluated from the economic perspective. Modeling the spectrum market system by using game theory, the convergence characteristics of the system are analyzed from two points of view: allocation interval and prediction period. The available spectrum is allocated at regular time intervals and the length of this allocation interval naturally has a significant impact on the dynamics of the system. Also, available spectrum is allocated based on the prediction of the time variation of the demand for wireless services. This prediction thus influences the dynamics of the system. Simulation results reveal that short allocation interval and long prediction period are required to improve the convergence property of the system. Simulation results also reveal that long prediction period leads to the increase in the revenue of spectrum holder.

The introduction of adaptive array antennas to wireless ad hoc networks is effective for increasing the overall network throughput because of its interference suppression capability. The network throughput can be further improved by controlling antenna-pattern, channel, and power simultaneously. In this paper, the asynchronous distributed pricing (ADP) and multi-channel ADP (MADP) algorithms, which are joint channel selection and power control schemes, are modified to also select antenna-pattern. Simulation results show that the proposed scheme achieves high network throughput.

Reliable detection of other wireless systems is crucial for the systems that share the same frequency band. In wireless communication channels, there is uncertainty in the received signal level due to multipath fading and shadowing. Cooperative sensing techniques in which wireless stations share their sensing information can improve the detection probability of other systems. This paper presents the field trial results of a cooperative sensing scheme with energy detection and hard decision cooperation. The results demonstrate that the cooperation of wireless stations can relax the stringent sensitivity requirement of energy detection.

The present paper focuses the application of the base station cooperation (BSC) technique in fractional frequency reuse (FFR) networks. Fractional frequency reuse is considered to be a promising scheme for avoiding the inter-cell interference problem in OFDMA cellular systems, such as WiMAX, in which the edge mobile stations (MSs) of adjacent cells use different subchannels for separate transmission. However, the problem of FFR is that the cell edge spectral efficiency (SE) is much lower compared to that of the cell center. The BSC technique, in which adjacent BSs perform cooperative transmission for one cell edge MS with the same channel, may improve the cell edge SE. However, since more BSs transmit signals for one cell edge MS, the use of BSC can also increase the inter-cell interference, which might degrade the network performance. In the present paper, with a focus on this tradeoff, we propose an adaptive BSC scheme in which BSC is only performed for the cell edge MSs that can achieve a significant capacity increase with only a slight increase in inter-cell interference. Moreover, a channel reallocation scheme is proposed in order to further improve the performance of the adaptive BSC scheme. The simulation results reveal that, compared to the conventional FFR scheme, the proposed schemes are effective for improving the performance of FFR networks.

This paper investigates the performance of a multiuser distributed antenna system (DAS) with spatial precoding in which mobile stations (MSs) just send back the received signals along with their training sequences. In this method, base stations (BSs) can obtain the channel state information of both the uplink and downlink without estimating the channels in MSs. This two-way multiple-input multiple-output (MIMO) channel estimation technique have been proposed and mathematically analyzed in the literature. However, implementation aspects of this method, such as forward gain and dynamic range, are not studied yet. In this paper, these aspects are investigated in order to confirm its feasibility in a multi-user DAS where a precoder is employed in BSs. Computer simulation results are presented to demonstrate the required hardware specs for an amplify and forward (AF) operation in MSs by examining the bit error rate (BER) performance. It is shown that an 8-bit A/D converter together with a fixed gain amplifier brings almost no degradation to the BER performance. Furthermore, a performance comparison with a vector feedback method is given.

A distributed power, channel, and route selection scheme for multi-hop radio networks is proposed by extending multichannel asynchronous distributed pricing (MADP) algorithm, which is a power and channel selection scheme for single-hop radio networks. MADP algorithm achieves suboptimal system throughput in single-hop radio networks, however, since MADP algorithm allocates more resources to links with good quality, there may be a large difference in throughput among communication links. Particularly in multi-hop radio networks, MADP algorithm might not necessarily achieve high system throughput because the link with the lowest throughput limits the end-to-end throughput of a given multi-hop route. In order to achieve high system throughput in multi-hop radio networks, a modification of MADP algorithm named end-to-end throughput aware MADP (ETA-MADP) algorithm is proposed. It is proved that all links in a given multi-hop route have the same throughput through the use of ETA-MADP algorithm. Simulation results reveal that the introduction of the proposed algorithm achieves high spectral efficiency in multi-hop radio networks even with lower transmit power compared to that in single-hop radio networks.

Multi-hop relaying can extend communication range using limited transmit power, and improve area spectral efficiency. However, they might cause considerable degradation of end-to-end error performance due to error accumulation. Cooperative relaying, in which multiple stations forward the received signal using transmit diversity techniques, can recover the errors unless all the relay stations do not receive the signal correctly, and improve the transmission performance. In this paper, the FER performance of multi-hop cooperative networks is analyzed by using state transition. Theoretical analysis reveals that the FER performance can be kept almost constant, or even improved with increasing the number of hops. The computer simulation results closely agree with the analysis.

Cooperative relaying is a promising technique that improves the end-to-end error performance in multihop vehicular networks. When cyclic delay diversity (CDD) is employed as transmit diversity at each hop, the control of the delay time introduced at each transmitting station improves the diversity gain. In this paper, a novel distributed delay time selection algorithm for cooperative multi-hop vehicular networks with CDD is proposed. In this algorithm, neither dedicated control packets nor an external controller for the delay time selection is required since the delay time is selected by overhearing the transmission in the next hop. Simulation results show that the proposed algorithm can achieve the end-to-end FER performance close to that of fixed delay time allocation, and is not sensitive to vehicles movements.

Cooperative relaying is a promising technique for multihop wireless networks to exploit spatial diversity. Most of the studies of multihop relaying assumed a simple i.i.d. Rayleigh fading model. In this case, there is no correlation of phase and amplitude among the received signals. However, this assumption is not always fulfilled in practice. For instance, the transmission distance in multihop wireless transmission is supposed to be a close range and the shadowing effect needs to be taken into account. The ray launching method based on geometrical optics is a technique for estimating a deterministic propagation channel. In this paper, 3D ray launching method is used for wireless propagation prediction. By computer simulations, the performance of 2-hop cooperative relaying with two relay stations is investigated. The simulation-based performance analysis confirms that the cooperative relaying scheme has an advantage of diversity gain thus improving the bit error ratio performance.

In this paper, the performance of decentralized adaptive route selection problem in cooperative relaying networks in which each source independently chooses a route to enhance its own throughput is studied. Cooperative relaying is a new technique to enhance spectral efficiency by using multiple stations. However, when multiple stations transmit simultaneously, it also increases the number of interference signals. For this reason, if each source independently and selfishly transmits, it is not clear whether cooperative relaying always increases the network capacity. In order to investigate the capacity of cooperative relaying under the decentralized adaptive route selection, the game-theoretic approach is applied. Simulation results reveal that the introduction of the cooperative relaying always increases the capacity under centralized control, however that it may reduce the capacity under decentralized control due to the increase in the number of interference signals.

In cognitive radio, reliable detection of the primary system is crucial for the secondary system. However, there is an uncertainty in the received power level due to shadowing. The cooperative sensing where the stations in the secondary system share their sensing information can improve the detection accuracy of the primary system. In this paper, a new secondary system for reducing interference to the primary system is proposed. In the proposed system, a secondary transmitter receives sensing information by using multi-hop transmission from all secondary stations which detect the primary system, and makes decision of its transmission based on the received sensing information. Moreover, it controls the transmit power by the received CINR information from the secondary sensing stations. Simulation results assuming spatially independent shadowing and ideal multi-hop transmission in the secondary system reveal that the proposed system is helpful to protect the primary receivers which have no transmission ability.

A cooperative diversity requires accurate synchronization for relaying in multihop communications. This paper presents an iterative equalizer for multihop networks with delay diversity, which is tolerant to asynchronous relaying. We introduce frequency-domain space-time coding suited for the equalizer. In addition, we employ cooperative spatial multiplexing to improve the system throughput. We demonstrate that cooperative diversity and multiplexing can offer high spectrum-efficiency even in frequency-selective and poor-scattering channels

Wireless multihop communications with the cooperation of multiple relay terminals at each hop gives diversity effect and shows a significant frame error rate improvement than that without cooperation. To improve the spectral efficiency of this communication networks, adaptive modulation method is applied in this paper. The performance of adaptive transmission schemes combined with diversity techniques in multihop cooperative networks is analyzed. Numerical results from the analysis show that the throughput performance of adaptive transmission combined with a selection diversity technique is superior to that of the STBC diversity technique.

In mobile multihop communications, the diversity technique is important since the channel between mobile terminals may suffer from the effects of fading and the routes between terminals are normally created through several hops. Space-time coding using multiple transmit antennas is an effective countermeasure for fading. For terminals which have only one antenna, due to the size/cost limitations, user cooperation has been introduced as a diversity technique. In this paper, a space-time coded cooperative relaying scheme for a multihop communications scenario, in cases where the number of hops is more than two, is proposed in order to combat the effects of fading among single antenna terminals, without bandwidth expansion. The scheme utilizes a space-time code for cooperative relaying and is developed in both frequency-flat and frequency-selective fading channels. By computer simulations, it is shown that the proposed scheme provides diversity gain and more improvement of frame error rate as the number of hops increases. Furthermore, the effects of an error correcting code and co-channel interference are shown.

An automatic-repeat-request (ARQ) scheme for improving the system throughput efficiency is investigated in coded multiple-input multiple-output (MIMO) transmissions. The improved ARQ scheme employs two strategies, referred to as trellis-coded modulation (TCM) reassignment and antenna permutation. TCM reassignment increases the free distance after code combining through the use of different TCM codes for retransmissions. Antenna permutation exploits spatial dimension for preventing successive frame errors by changing the connections between transmitters and transmit antennas upon retransmission. It is shown through computer simulations that this ARQ scheme achieves high throughput even in severe conditions of low signal-to-noise ratio and high Rician factor over spatially and temporally correlated Nakagami-Rice fading MIMO channels.

Multiple input multiple output (MIMO) transmission techniques hold the potential of dramatically increasing the data rates and spectral efficiency of wireless communication systems. However, since multiple transmitters and receivers are required, a large number of channel emulators are also required for in-lab experiments, resulting in an increase in equipment cost. This paper describes an implementation of received signal generator for a 4/spl times/4 MIMO transmission in one FPGA chip which can be utilized as a real-time MIMO spatial multiplexing system test-bed. Our experimental results demonstrate that the CDF and channel correlations of emulated Rayleigh fading under this fading channel approach the theoretical values.

Multiple-input multiple-output (MIMO) transmission systems, exploiting multiple antennas at both Tx and Rx sides, have potential to dramatically increase the spectral efficiency in rich-scattering wireless links. At the receiver, the so-called space-time equalizer can adaptively deal with dispersive channels and jointly decode multiplexed signals. Recently, low-density parity-check (LDPC) codes have drawn lots of attention for achieving excellent error-correction performance by means of the low-complexity belief propagation (BP) algorithm. This paper explores an iterative space-time equalizer employing soft cancellation followed by frequency-domain linear filtering, and evaluates the error-rate performance in LDPC-coded MIMO transmissions over time-varying frequency-selective fading channels.

The performance of the turbo equalization algorithm combined minimum mean square error (MMSE) filter with low-density parity-check (LDPC) codes is investigated under frequency selective fading environment. Spatial multiplexing scheme can improve frequency efficiency without channel knowledge at the transmitter. In this system different users transmit LDPC encoded information on the same carrier frequency at the same time. The receiver separates multiplexed signals into individual signals of each user by MMSE filter. After that, each information bit is decoded by sum-product algorithm and log likelihood ratio (LLR) of them are updated. The soft replica is generated from its LLR. Interference components are subtracted from the received signal. The MMSE filtering is reapplied to the resultant signal. This paper shows the performance of this scheme in 4x4 multiple-input multiple-output (MIMO) system and reveals the effect of iteration and channel estimation on the bit error rate (BER) performance.

This paper evaluates the effect of spatial interleaving in the Viterbi algorithm with combined intersymbol interference (ISI) equalization, trellis-coded modulation (TCM) decoding and multi-signal detection. Through computer simulations, it is confirmed that spatial interleaving provides substantial diversity gains in both frequency-flat and frequency-selective Rayleigh fading channels. In Nakagami-Rice fading channels, performance improvement is significant at low Rician factor because of overcoming the burst errors incurred by signal fading. Spatial interleaving does not require additional computational complexity, memory buffer and decoding delay, differently from time interleaving. Moreover, the optimum combined TCM and ISI maximum likelihood sequence estimation (MLSE) can employ interleaving with almost no extra complexity, and iteration between equalization and decoding is not needed through the use of spatial interleaving.

This paper presents a performance comparison between space-time coding (STC) and trellis-coded cochannel interference canceller (TCC) assuming both frequency flat and frequency selective fading channels. The STC is a form of a transmit diversity scheme as it transmits multiple signals using different antennas in order to carry a single data stream. The TCC is a form of multiuser detector, which can estimate multiple signals from different transmitters. This paper describes the difference between STC and TCC, and then provides the BER performance of them over frequency flat and frequency selective fading channels. It is shown by computer simulations that TCC is as good as STC when channel impulse response is estimated, and that TCC can outperform STC over frequency selective fading channels.

This paper proposes a multi-hop radio access cellular (MRAC) scheme for achieving both high speed and high capacity and good area coverage in fourth generation mobile communications systems. In this scheme, we assume two kinds of hop stations, one is a dedicated repeater station installed at a good propagation location such as a rooftop, and the other is a user terminal that temporarily experiences good propagation conditions. For both cases, the path diversity effect can be obtained between single-hop and multi-hop paths. Four propagation models were studied representing the paths among the base station, hop stations, and mobile station. Based on the propagation models, suitable MRAC operation conditions are discussed. For mobile stations located at the cell edge area, MRAC operation is effective in reducing the transmit power. Consequently, MRAC reduces the interference in cellular systems and enhances the area coverage.

In recent years, a lot of attention has been drawn to the use of multiple antennas at both the transmitter and receiver sides. Such techniques are named here as space-time transmission and noted as STT. By their use of the available space dimension, STT techniques can achieve very high spectral efficiencies in Rayleigh fading channels with rich scattering. The use of STT techniques for indoor wireless systems with a huge number of antennas is considered. As a low complexity detection algorithm, ordered successive detection based schemes are considered. A scheme known as V-BLAST (vertical Bell Labs layered space-time) is proposed in the zero forcing (ZF) criterion (see Wolniansky, P.W. et al., Proc. ISSSE'98, 1998). In a previous work, it has been shown how this scheme can be much improved by the use of a backward iterative detection algorithm (see Benjebbour, A. et al., Proc. WPMC, p.819-24, 2000; Proc. VTC2001 Spring, 2001). An extension of the conventional V-BLAST algorithm to the minimum mean square error (MMSE) criterion is now proposed. For the proposed receiver, an ordering metric is defined. Then, the effect of ordering on both receivers is discussed. And finally, it is shown how the ordering metric in the MMSE criterion can be simplified.

Cochannel interference is a major impairment that limits the capacity of cellular radio systems. A coding scheme for nonlinear cochannel interference cancellation is discussed. This canceller uses trellis-coded modulation with maximum-likelihood sequence estimation of both the desired signal and cochannel interference. This technique permits operation even when the signal-to-interference ratio is 0 dB. This performance advantage is due to the sophisticated trellis-coding designed for each user. A novel trellis coding is proposed, and its superior bit error rate performance is shown by computer simulations.

A new hybrid automatic repeat-request (ARQ) scheme that uses supplementary trellis codes for retransmission is described. In this scheme, supplementary codes are designed to compensate for the squared free distance (d/sub free//sup 2/) of the trellis code used for the previous transmission. Differently encoded codewords are combined and decoded by the same Viterbi decoder at the receiver. This scheme provides improved (d/sub free//sup 2/) upon retransmission compared with that provided by the conventional trellis coded hybrid-ARQ scheme where the same coding scheme is used for all transmissions.

Co-channel interference is a major factor limiting spectral efficiency of cellular radio systems. A nonlinear co-channel interference canceller leading to a significant increase of traffic capacity of cellular systems has been investigated. In this scheme, a maximum-likelihood sequence estimation extended to cancel co-channel interference is combined with trellis-coded modulation to enhance the co-channel interference cancelling capability. However, the bit-error rate performance of this canceller is degraded if there exists a frequency offset between the desired signal and the interference signal. In this paper, a joint frequency offset and delay profile estimation technique for a nonlinear co-channel interference canceller is proposed. In this algorithm, frequency offset is simultaneously estimated during delay profile estimation using the residual error of delay profile estimation. Computer simulations have been performed to demonstrate the performance of the proposed algorithm. It is shown that the allowable frequency offset range of the proposed canceller is over 10 times as large as that of the conventional canceller without frequency offset compensator.

Cochannel interference is a major impairment that limits the capacity of microcellular radio systems. This paper proposes a trellis-coded cochannel interference canceller, which uses trellis-coded modulation with maximum likelihood sequence estimation of both the desired signal and cochannel interference. This technique permits operation even when the signal-to-interference ratio is 0 dB. Computer simulation results show that this technique significantly reduces the bit-error rate in microcellular systems with Nakagami-Rice fading, particularly when the average signal-to-interference ratio is 0 dB. However, the technique requires: (1) synchronization of the interference and the desired signal and (2) higher computational complexity than standard techniques.

It is highly desirable to develop an efficient and flexible dynamic channel assignment algorithm in order to realize an integrated traffic TDMA mobile radio communication network. In this paper, an integrated traffic TDMA system is studied in which transmission of voice and data are assumed to occupy one and n time slots in each TDMA frame, respectively. In general, two types of channel (time slot) assignment algorithms are used: the partitioning algorithm and the sharing algorithm. However, they are not well-suited to the multimedia traffic consisting of various information sources that occupy different number of slots per frame. In this paper, assuming that voice is much more sensitive to transmission delay than data, an algorithm based on the sharing algorithm with flexible time slot management scheme is proposed as follows. To reduce the blocking and forced termination probabilities of a voice call, part of the slots of data calls are transferable to voice calls during congestion periods. At the same time, to reduce the handoff-failure of a multi-slot data call, the required number of data slots can be reduced adaptively according to the available number of slots during its handoff. These data calls whose number of slots are reduced either for voice calls or its handoff are allowed to get restored when possible. Computer simulations confirm the good performance of the proposed algorithm when compared to conventional channel assignment algorithms.

Trunk transmission network systems connecting exchange offices were the first transmission systems using optical fiber cables. Subscriber network systems connecting the exchange office with each subscriber including local area networks or CATV network systems are under development at present. Two main techniques are newly required to be developed for making optical fiber mechanical connectors for optical fiber subscriber network system realization. The first is to realize a new connector with a high return loss and a low connecting loss. The second is to cut the joining cost of optical fibers. The newly developed "optical contact connector" realized a high return loss of more than 25 dB and a low connecting loss of 0.15 dB average value. A newly developed "multifiber connector" which can connect many optical fibers at the same time can save time and space for joining fiber cables, and also a "plastic molding technique" can save the cost of the connector. The performance of all these connectors is the same as that of the "optical contact connector." This paper describes the present status of the technique to obtain a high return loss connector, a multifiber connector, and a plastic molded connector for the subscriber network system.